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- 7 MAY 1987
BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS
RECORD
BHR Record 1987/l.1
In-situ Stres s Measurements With the Hydraulic Fracture Technique
P.N. Chopra and J.R. Enever
3 The information contained in mi. report ha. been obtained by the Bureau of Mineral Reaourc •• , Geology and GeophyalC8 ••
l-\, . part of the policy of the Auatralian GovernlTl8ft to a •• i.t in the exploration and development of mineral reaourc ••. It may not be .j --- publi.hed in any fonn or u.ed in a company pro.pectu. or Itatemant without the pannl .. ion in writing of the Director.
I I I
~ t t
In-situ Stress Measurements With the Hydraulic Fracture Technique
+
+ P.N. Chopra and J.R. Enever
BMR Record 1987/1]
CSIRO Division of Geomechanics
Kinnoul Grove
Synda1 Vic 3149
Table of Contents
Page Number
Abstract iii
Introduction 1
Equipment 2
Experimental Procedures 5
Test Sites 6
Berrigan, NSW 6
Lancefield, Vic 8 Wong an Hills, WA 8-
Results of Field Tests 8
Results of Laboratory Tests 14
Discussion 18
I Conclusions 21
Acknowledgements 22
Bibliography 23
i
, !
Abstract
A new hydraulic fracture system has. been developed and constructed
for use in the measurement of in-situ crustal stress. This system
incorporates a down-hole flow meter, a new development which allows
much more precise resolution of crack re-opening pressures and greatly
aids in the interpretation of the hydraulic fracture pressure records.
Hydraulic fracture stress measurements have been made in granitoids at
3 sites in Australia at which independent measurements of the in-situ
crustal stress had already been obtained with th~ overcoring method.
In all cases, the results of the hydraulic fracturing and overcoring
methods are consistent. This suggests that both methods can reliably
determine the stress in the Earth's crust. In the hydraulic fracture
tests reported here, two types of induced fracture geometries have
been observed: fractures in the test section and fractures under the
sealing packers. Each fracture geometry results in a distinctive
pressure record. With fractures of the first type, it is shown that
use of crack re-opening pressure in the analysis provides reliable
estimates of the in-situ stress. With fractures under the packers on
the other hand, it has been found that analysis of the results based
on crack initiation pressure and the mean laboratory-determined rock
strength results in reliable in-situ stress estimates.
iii
~ .
1. Introduction
The Commonwealth Scientific and Industrial Research Organisation
(CSIRO) and the Bureau of Mineral Resources, Geology and Geophysics
(BMR) have been cooperating for a number of years in a program of
regional stress measurements in granite outcrops throughout Australia
(Denham et al, 1979; Denham et a1, 1980; Denham and Alexander, 1981).
The aim of this work has been to investigate the magnitude,
orientation and distribution of tectonic stress within the continent,
to study the relationship between this stress field and large scale
geological structure and, to obtain data for the assessment of
earthquake risk.
Initial efforts involved a number of overcoring techniques
including the use of United States Bureau of Mines (USBM) borehole
deformation gauges and. the CSIRO triaxial borehole deformation gauge.
These overcoring measurements were made in granite outcrops in close
proximity to the surface (generally less than 5 metres depth) at a
number of locations throughout Australia (loc. cit.). Where the
results obtained are considered to be reliable, they have generally
indicated a NE-SW or E-W orientation of the maximum horizontal
principal stress.
These results, while they have served to delineate some broad
features in the stress field, have also pointed to three serious
limitations inherent in the overcoring technique in its application to
Australian conditions.
1) The need to carry out overcoring at relatively shallow depths
(i.e. less than a few tens of metres) is an important limitation in an
Australian context because of the deep weathering profiles that
characterise the near surface rocks throughout much of the continent.
Such deep weathering can compromise the relevance of near surface
measurements because of the possibility of decoupling of the near
surface rocks from the underlying crust both through the greater
deformability of the weathered material and as a result of the
presence of associated open cracks and joints.
2) Near surface stress measurements are also likely to include
1
stresses associated with diurnal and seasonal temperature changes that o
can be as great as 50 C.
3) Such measurements can be influenced by topographic effects
including variations in the cover of weathered material (Denham and
Alexander, 1981).
In view of these factors, it was decided to substitute a stress
measurement program based on hydraulic fracture because this
technique offers the opportunity of making in-situ stress
determinations at much greater depths in fresh tectonically-coupled
rock. The application of hydraulic fracturing to the stress
measurement program in hard rocks has drawn on extensive
experience with hydro fracture obtained by CSIRO in sedimentary basin
rocks (see for example, Enever and Wooltorton, 1983) and hence, the
design philosophy of the down~hole tools and the instrumentation that
is reported here is broadly similar to that used previously by CSIRO.
The initial work with the hydrofracture system has been carried
out at three sites: Berrigan (NSW) , Lancefield (Vic) and Wongan Hills
(WA). At each of these sites overcoring measurements had previously
been made in the relatively fresh, high modulus rock which outcrops at
the surface.
2. Equipment
A hydrofracture system, comprising a fracture tool and an
-impression tool , has been designed and constructed specifically for
this work. Figures 1 and 2 show the essential components of the
fracture tool, together with the handling system and the up-hole
hydraulic and electronic equipment. The tool is connected to the
surface control system by means of two flexible high pressure hoses
and a seven conductor armoured co-axial cable. The former allow
independent connection of the sealing packers and the isolated test
interval (horizon) to separate hydraulic pumps housed in the control
vehicle, while the latter provides the necessary electrical and
mechanical connections. Important components of the fracture tool
include two transducers for the measurement of pressure in the packers
2
1
r·
TRIPOD
WIRELINE / DATA CABLE
PACKERS PRESSURE mANSDUCER
SOLENOID DUMP VALVE -----~ ~----
TEST HORIZON
FLEXIBLE HYDRAULIC HOSES
Q,.. ,+-.---1-4:> '. ,,:, iI--1--io __ - __ 1- ------r-....-~_.J
TEST SECTION PRESSURE TRANSDUCER
FLOW METER
INFLATABLE PACKERS
Figure 1 Schematic diagram of the hydrofracture system.
3
Figure 2a
Figure 2b
The fracture tool, handling system and
hydraulic components.
The interior of the truck-borne instrument cab
showing the hydraulic and electronic controls.
4
and the test interval, a flow meter for the measurement of fluid
movement into or out of the test interval, and a solenoid operated
dump valve to permit effective deflation of the packers at the end of
the tests. The length of the test interval can be varied to suit test
conditions by using different length spacers (currently 0.5 and 1
metre spacers are used).
The impression tool used in conjunction with the fracture tool
includes a digital compass which provides an up-hole display of the
tool's orientation in the hole. This arrangement provides a reliable
means of determining the orientation of the induced fractures at the
borehole wall.
3. Experimental Procedures
The hydrofracture field tests have been carried out at each site
in a similar fashion. Water has been used as the fracture fluid and
the rates of pressurisation have been relatively slow (approximately
sMPa per minute). The field test procedure followed has involved an
initial fracturing phase followed by a number of cycles of
pressurisation and subsequent venting of the test interval. The
initial fracturing is accomplished by concurrently but independently
increasing the pressure in the packers and the test interval. In this
way a small positive pressure differential can be maintained between
the packers and the test interval which helps confine the fluid in the
latter. Pumping is stopped as soon as a crack is formed in order to
preserve the initial geometry and to allow a first shut-in pressure to
be recorded. The test interval is then vented to atmosphere to allow
the induced crack to close. A build-up of pressure in the test
interval upon temporarily sealing the system during venting ("pressure
rebound") is taken as evidence of continued flow of water out of a
closing crack. Venting is continued until this phenomenon ceases.
Further cycles of pressurisation and venting are used to determine
the crack re-opening pressure and to gauge whether the orientation of
the crack changes as it is propagated. Such a change of crack
orientation is reflected in systematic changes in shut-in pressure
5
from cycle to cycle. Crack re-opening pressure is determined with the
aid of the down-hole flow meter.
4. Test Sites
In this record, discussion will be limited to three of the sites
where hydrofracture stress measurements have been made in granitic
rocks (see Figure 3). These sites characterise two different types of
results we have obtained in our stress measurement program, both in
terms of the form of the pressure-time data recorded and the nature of
the fractures produced. These sites are also of particular value
because independent estimates of the deviatoric stress state are
available from overcoring measurements made nearby (less than 10m
laterally distant). These sites are
1) Berrigan, New South Wales
The measurements were made in a borehole drilled in a granite
of Upper Silurian age at depths between 4 metres and 167
metres. The borehole is located in a small disused quarry in
flat lying country in southern New South Wales on the western
flank of the Lachlan Fold Belt. The quarry is located at grid
reference CD93l5l0 on the 1:100,000 Berrigan Topographic
Sheet. The granite is a coarse grained rock with an average
grainsize of 1670 microns (as determined from thin section by
the linear intercept method of Exner (1972) using a
sectioning correction factor of 1.5) which contains crystals
of orthoclase of up to 2 cm. In thin-section the rock is seen
to be essentially free of deformation induced microstructures
other than a few non-pervasive microcracks which are located
principally along grain boundaries. Ample evidence in the
form of pop-ups and stress controlled jointing exists in the
Berrigan area which suggests that the ground is currently
being subjected to a strong horizontal compressive stress
field. This is of particular interest because the area has a
history of seismicity with a magnitude 5.5 earthquake
recorded in the vicinity in 1938.
6
N
BERRIGAN S
WONGAN HillS S LANCEFIElD S
ORIENTATION OF THE MAXIMUM PRINCIPAL STRESS INFERRED FROM HYDRAULIC FRACTURE MEASUREHENTS
- - INFERRED FROH OVERCORING MEASUREHENTS
-
Figure 3 Location map showing the test sites and the
orientation of the maximum principal stress
as deduced from hydrofracture and overcoring
measuremnents.
7
2) Lancefield, Victoria
This site is located in a broad valley in gently rolling
country near the town of Lancefield approximately 75 o
kilometres north of Melbourne at 37 II'S latitude and o
144 45'E longitude. The borehole penetrates a fine grained
undeformed graniodiorite of Upper Devonian age with an
average grainsize (determined as above) of 1440 microns. This
site is located within the Lachlan Fold Belt. The
hydrofracture measurements were made at this site at a depth
of 10 metres.
3) Wongan Hills (Cousin's), Western Australia
This site is approximately 150 km north-east of Perth and
within an area bounded by the towns of Meckering,Calingiri
and Cadoux, the sites in recent years of earthquakes of
magnitudes 6.9, 5.9 and 6.5 respectively. This area is part
of the Yilgarn Block, a multiply deformed Archaean craton. o
The borehole is sited at latitude 30.87 S, longitude o
116.96 E. The rock penetrated by the borehole is a deformed
rock of granodioritic composition which has probably
undergone recrystallisation during its deformation history
and is now relatively fine grained. The average grainsize
determined from thin-section is 980 micron. The hydrofracture
test results discussed here are reported in detail elsewhere
(Denham et aI, 1986 in prep.).
5. Results of Field Tests
Vertical hydraulic fractures were initiated during testing at each
site. At Berrigan, the fractures were found generally to correspond
with the test interval, extending at each end into the regions occupied
during testing by the packers. In contrast, at the Lancefield and
Wongan Hills sites the fractures were found to be restricted to the
zone occupied during testing by one or other of the inflatable
packers. The orientations of the fractures induced at the three sites
are shown in Figure 3.
8
1
I I
Also shown in Figure 3 are the corresponding orientations of the
major horizontal stress component determined by overcoring tests at
each site. For all these sites, the agreement of orientation between
the overcoring and hydraulic fracturing stress measurements can be
considered good, if the orientation of the induced fracture is taken
to represent the orientation of the major horizontal stress component.
This is true irrespective of whether the induced fracture formed in
the test section or under the packers.
The difference in the locations of the fractures induced at.
Berrigan compared to Lancefield and Wongan Hills was reflected in the
form of the pressure records obtained from the respective sites.
Figure 4a is typical of the pressure records obtained for the majority
of the tests conducted at Berrigan (i.e. excepting the test at 4m
depth). This record exhibits a distinct and sharp crack initiation,
well-defined and consistent shut-in pressure for successive cycles of
pressurisation and noticeable pressure "rebound" during venting of the
test section between cycles. The data in this figure also display'
clear indications of crack re-opening during the repressurisation
cycles (defined by the onset of measureable fluid flow through the
down-hole flow meter) and point to a fairly constant fluid pressure
during crack propagation. In these respects, the data of Figure 4a
might be considered a "text-book" example of a hydrofracture test.
Analysis of the Berrigan field data has in each case been based on
the conventionally accepted use of shut-in pressure as the minor
horizontal stress component (~ ) and crack re-opening pressure for h
calculation of the magnitude of the major horizontal stress component
(~ ). It has also been assumed that these horizontal components of the H
stress field are principal stress components and that the third
principal component (~ ) is therefore vertical and coaxial with the 3
test hole. The methods used to select the salient data from the test
results are illustrated in Figure 4a. These data are tabulated in
Table 1, along with a summary of· their analysis based on the
relationship:
9
,... o
Il.. 20 :::E: -..;
w a::: ::J 10 (J) (J) w a::: Il..
,... 12 o
Il.. :::E: -..; 8
o 5
o 5
Figure 4
10 15
10 15 20
20 25 30 TIME IN MINUTES
Z ;I => :c VI
I.U VI => u.. u.. o
~
Fig. 4 (a) •
:.::: u « 0::: WI.:J
f-Z wZZ - LU f-o.. VlO 0 1
~~
+
25 30 3S 40 TIME IN MINUTES
Fig. 4(b).
35
45
) ( WELL DEFINED SHUT-IN PRESS URE
o (JISTINCT CRACK RE-OPENING PRESSURE (DEFINED BY ONSET OF FLOW RECORDED HY DOWN -HOLE FLOWMETER)
40
c::------ PACKER PRESS.
--":'~--TEST SECTION PRESS.
45 50
PRESS.
TEST SECTION PRESS.
70 75 80
Abridged test records showing the important features and
the methods used to extract the data. Fig. 4 (a), Berrigan;
Fig. 4 (b), Lancefield.
10
TABLE 1. SUMMARY OF DATA FOR BERRIGAN SITE ---_._---_._--- -----_.
Depth of Estimated Crack Initiation Ranqe of Shut-in Ranoe of Estimate of Estimate of Orientation of at Test Pore Pressure (Pi) MPa Pressures (1st qe-oneninn a, ~~a at ~Pa Horizon Pressure (Correspondinq cycle-nth cycle) Pressures
(m) (Po) MPa packer press.) MPa (Pr) MPa
4-5 0 11.6 (14.2) 5.6-5.B-6.0 5.7-6.0 5.6-6.0 11.1-12.0 69-70 0.7 15.5 (16.2) 5.6-5.5-5.3 5.6-5.5 5.3-5.6 9.7-10.5 70" East of True Nth
100-101 1.0 17.4 (18.9) 6.0-5.5-4.9 6.3-5.6 4.9-6.0 R.1-10.7 75" East of True Nth
125-126 1.3 18.5 (19.7) 6.3-6.2-6.1 6.7-6.7 6.1-6.3 10.1-10.9 70· East of True Nth
154-155 1.5 20.7 (21.5) 6.3-6.1-5.7 7.0-7.0 5.7-6.3 8.6-10.4 72' East of True Nth
168-169 1.7 18.6 (19.0) 7.7-8.0 8.4 7.7-8.0 13.0-13.9 73' East of True Nth
Results from Overcorino in Ad.iacent Hole ._---------
3.5 5.6 11.4 74" East of True Nth 3.8 9.5 12.1 80' East of True Nth
TABLE 2. SUMMARY OF DATA FROM LANCEFIELD AND \,ONGAN HILLS SITES
Depth of Test Horizon
Estimated Pore Pressure (Po) MPa
Crack Initiation Pressure (Pi) MPa (Corresponding packer press.)*
Ranqe of Shut-in Pressures ( 1st cy~le-nth cycle)
Ranoe of Re-ooenino Pressures (Pr) ~Pa
Laboratorv S trennth . Ranqe, MPa
Estimate of a. MPa
Estima te of (jl** tJlPa
(Averaoe)
Orientation of at
(m)
approx. 10m
Average of two tests at approx. 10m
66-67
69-70
6.5 10.5
° 10.9 (11.6)
0.7 9.5 (9. {,)
0.7 11.4 (11..5")
MPa (Averaoe)
Lancefie1d 7.0-3.9-3.9-3.9-3.9-3.9
4.0-4.7.- 10.0-12.0 4.2-4.2-4.6 (11.0)
Results from Overcorino in Adiacent Hole
Y/onqan Hills 6.3-5.1-4.3- 4.2-3.9- 13.2-15.0 4.3 3.9 (14.1) 5.5-6.1-6.6- 3.9-3.9- 13.2-15.0 6.6 4.2 (14.1 )
Results from Overcorina in Adjacent Hole
* Pressure in packers at crack initiation. used in analysis ** K = 1.0 used in analysis
II
3.9
3.4
4.3
6.6
3.4 6.8
10.1-12.1 (11.1)
11.0
15.9-17.7 (16.8) 20.9-22.7 (21.8)
18.2 20.4
68' East of True Nth
78' East of True Nth
79' East of True Nth
60" East of True Nth
62" East of True Nth 87" East of True Nth
a 1
3.a - K.P - (2-K).P 2 r 0
(Haimson, 1978)
where: a is estimated directly from the shut-in pressure 2
P is the crack re-opening pressure r
P is the ambient pore pressure at the location of the o
test horizon (based on the assumption that the
water table is at the surface)
and, K is a poro-elastic constant which is generally assumed
to be equal to 1.
(1)
Also included in Table 1 is a summary of the results of the overcoring
tests conducted at the Berrigan site.
The data in Table 1 show a high degree of consistency, both in
terms of the relatively small ranges observed for shut-in pressure and
re-opening pressure for any given test, and in terms of the general
agreement between the results for the six hydrofracture tests. There
is also good agreement between the hydro fracture tests and the
overcoring results.
Figure 4b, containing data obtained from the Lancefield site, is
generally representative of the pressure records obtained for the
tests at Lancefield and Wongan Hills, and for one test at Berrigan (at
approximately 4 metres depth). The distinctive features displayed in
Figure 4b include a much more rounded crack initiation phase, more
variable and poorly defined shut-in phases and a lack of any
detectable pressure rebound during venting of the test section between
tests. Crack re-opening is indistinct and significantly, there is a
pronounced hump in the first re-pressurisation cycle. All of these
features can be linked to the initiation of fracturing under the
sealing packers rather than in the test interval.
The less distinct crack initiation and more variable and poorly
defined shut-in phases apparent in Figure 4b result from the fluid in
the test section having to leak past the inflated packer to enter the
crack. This restriction of fluid access results in a dampening of the
test interval pressure response. The lack of significant pressure
rebound and of a distinct re-opening pressure can be attributed to the
12
propping effect of the packer on the crack initiated under it. The
distinct hump in the pressure-time record presumably represents the
excess pressure required to extend the original crack along the length
of the hole to a position where fluid from the test section can gain
direct accesS to it.
It is also notable in Figure 4b that the pressure responses during
the tests subsequent to the first re-pressurisation are very similar.
The pressure~time records for the crack propagation phases and the
shut-ins are essentially identical. This observation is indicative of
the diminishing influence of the original crack location as crack
propagation proceeds. The only significant difference between the
later cycles is the somewhat higher crack propagation pressure value
in the last test. This elevated pressure is linked to the higher
packer pressure used in this test and may reflect reduced fluid access
to the crack during propagation.
The absence of distinct re-opening pressures for the data obtained
from the Lancefield andWongan Hills sites necessitated the use of a
different approach to analysis in these cases. As with the Berrigan
data, shut-in pressure was used to estimate the magnitude of (I • The 2
magnitude of (I was determined from the relationship: 1
(I
1 3.a + S - K.P - (2-K).P
2 i 0 (Haimson, 1978)
where: S is the fracture strength and,
P is the crack initiation pressure. i
(2)
Core recovered from the test holes at the Lancefield and Wongan
Hills sites was tested in the laboratory to determine appropriate
values of Sand K for each site. The outcome of this laboratory
testing is discussed in detail in section 6. Table 2 gives a summary
of the field and laboratory data for the Lancefield and Wongan Hills
sites, and a precis of the overcoring results.
In general terms, the data in Table 2 reflect fairly good
agreement between the hydraulic fracture and overcoring results,
though the measurements were made at quite different depths. This
13
correspondence is best when the average measured laboratory
strength is used. This agreement is evidence of the reliability of
both methods of stress measurement and of their ability to determine
the in-situ stress field. This holds even when the fractures initiate
under the packers, provided that the crack initiation pressure and
laboratory measured strength are used in the analysis rather than the
re-opening pressure. Scrutiny of the data in Table 2 suggests that use
of re-opening pressures would in all cases however have led to serious
under-estimations of the value of a . 1
It would appear from the data contained in Table 2 that when crack
re-opening occurs, it does so at a pressure closer to the value of a 2
rather than the value of 3a - a as generally assumed for the 2 1
analysis of crack re-opening pressures. This divergence from normal
behaviour may be due in part to the tendency of the packers to hold
the cracks open. Alternatively, the low re-opening pressures at the
Lancefield and Wongan Hills sites may result from the particular
stress states existing there. In both these cases, the net horizontal
stress fields tend toward a situation where a > 3.a . Under such a 1 - 2. .
stress state, it might be expected that a crack, once initiated in the
borehole wall, would not reclose with depressurisation.
The other notable feature of the data contained in Table 2 is the
larger range of shut-in pressures found at the Lancefield and Wongan
Hills sites than were found at Berrigan (Table 1). This greater
scatter can probably be attributed to variations in the influence of
the iriflated packer on fluid access to the crack as each test
proceeds. Since, as already discussed, the influence of the packer
decreases as the crack is propagated further however, it follows that
shut-in pressures in the later pressure cycles of a test are likely to
be provide more meaningful stress estimates.
6. Results of Laboratory Tests
The core samples from the Lancefield site were all tested without
external load. This testing strategy was chosen in order to simulate
the stress condition in the borehole wall during the hydro fracture
14
1
1
r I , I-
~
, ~ ,- 15-0
", a.
~ l: u .. i" a:: ~ VI VI w 10-0 a:. Q.
z a I-« I-
~ Z
~ ::::.::::: 5-0 LJ « ex: LJ
o
•
.'
Figure 5
• •
• • •
5 10 15 PRESSURISATION RATE, MPa/min
Summary of the laboratory results for the
Lancefield granodiorite (see text).
15
ro 0... L
.... w ex: ::> V) V) UJ cc c..
z 0
~ I--Z
~ u «
15-0 SLOPE APROXIMATELY = 1· 0
•
•
cc 10· 0 L-_--'--_----'-__ --'----Jo-----J'"--_--'
U 0 -5-0 TENSILE STRESS OUE TO EXTERNAL
LOAOfNG, 'MPa
Figure 6 Summary of the laboratory results
for the Wong an Hills granodiorite
(see text).
testing (i.e. a - 3.a in equation (2». The results of the these 1 - 2
tests which are presented in Figure 5, do not suggest a systematic
variation of crack initiation pressure with the rate of pressurisation
used in the tests. The limits of strength used in the analyses of the
Lancefield field data are indicated by the horizontal lines in the
figure. In the absence of specific laboratory data, a value of K = 1.0
was assumed for the analyses, based on general practice (Haimson,
1978).
In the case of the material from the Wongan Hills site, samples
were tested under external diametric loading in order to produce a net
tensile stress at the point of crack initiation in the interior of the
samples. This procedure was used in order to simulate the in-situ
stress field existing during the hydrofracture experiments which
produced a similar circumferential tension in the borehole wall at the
location of crack initiation (i.e. a > 3.a ). Figure 6 illustrates 1 2
the results of these laboratory tests on the Wongan Hills material.
Crack initiation pressure is plotted here against the calculated
uniform transverse tensile stress existing a short distance away from
the central hole in the specimens as a result of the external
diametric loading (Jaeger and Hoskins, 1966). The data in Figure 6 are
not inconsistent with an approximately linear relationship between
crack initiation pressure and tensile stress and fall within a
reasonably well defined band. The trend of this band is adequately
described by a value of K = 1+0.25 (from equation (2) with 3a - a - 2 1
assumed equal to the tensile stress and PO). The customarily used o
value of K=l was used in the analysis of the Wongan Hills field data,
together with the strength measured from the samples tested without
external loading (see Figure 6).
17
7. Discussion
The most obvious aspects of the field results evident from the
data contained in Figure 3 and Tables 1 and 2 are the apparent
orientational consistency of the horizontal s'tress field over vast
distances (probably coincidental) and the suggestion, based on the
Berrigan and Wongan Hills data, of a small systematic gradient in the
horizontal stress magnitudes with depth (see Figure 7). The overcoring
results from all three sites indicate that the near surface layers are
well-coupled to the tectonic basement.
From the viewpoint of the hydraulic fracturing technique, an
interesting feature of the experience gained at the three sites has
been the variation in the location of the induced fractures relative
to the tool geometry (i.e. under packer versus in the test interval)
and the commensurate implications on analysis of field data in
general. While the practical implications of crack location have been
treated above, the factors influencing the variation warrant further
discussion.
Two different explanations for the differing locations of crack
initiation are suggested by the results presented in this record, viz.
control of crack initiation by the petrophysical properties of the
rocks at each site, and control by the prevailing stress fields.
1) Petrophysical control
For the field tests conducted at Lancefield and Wongan Hills, the
packer pressure during initial pressurisation was kept only marginally
above the test interval pressure. Thus at crack initiation, there was
only a very small excess pressure in the packers. The most plausible
petrophysical explanation for the fracturing to have initiated under
these packers then is to suppose that the value of the poro-elastic
constant K must have been very close to 1. If the value of K had been
significantly above 1, cracks would have been expected to have formed
preferentially in the test section where the rock was exposed directly
to the influence of fluid penetration. It is notable that a value of K
-1 is also consistent with the results of the laboratory tests carried
18
,
50 -Q) J.....
4-J Q)
E - 100 J::
I 4-J
I 0. Q)
0 150 ,
~ ~
200
cr (MPa) H
5 10 15 20 ~, .. "",
~ '. \ \
\ . \ \
, . \ .
\ ..... . \ . \
. \0-
. I ' . .
\ V . ,
\ ........... \
. "
\ \ ~
\ '. \
• n
\ \ .
Figure 7, Plot of Maximum horizontal principle
stress versus depth at Berrigan,-NSW
~ hydrofracture results
• overcoring results
.rr vertical str,ess due ,togravi ty 'v
19
out in the tensile stress regime with this material.
In contrast to the situation at the other two sites, at Berrigan
even though the packer pressure during the initial pressurisations was
allowed to exceed the test interval pressures by on average 0.9 MPa in
the deeper tests, cracks still initiated preferentially in the test
sections. It was only when the packer pressure differential was
allowed to reach 2.6 MPa in the test at 4 metres depth that crack
initiation under the packers resulted. These field observations
suggest that the value of K for the Berrigan material might be
somewhat greater than 1. This conclusion will be tested in a
forthcoming series of laboratory experiments.
2) Stress control
The alternative explanation for the differing locations of the
crack initiations rests on the observation that the prevailing in-situ
"
stress field at Berrigan is very different from that recorded at the ~ Lancefield and Wongan Hills sites. At Berrigan, the state of stress in
the rock is such as to produce a net circumferential compression in
the borehole wall. In contrast, at the other two sites the stress
fields are characterised by a net circumferential tension (Wongan
Hills) and an approximately zero circumferential stress (Lancefield).
These differences may imply somewhat different failure criteria in
stress fields with very large stress ratios (i.e. u > 3.u ) compared 1 2
with more balanced stress fields such as at Berrigan (i.e. u < 3.u ). 1 2
Further work is now being undertaken to determine the nature
dfthe mechanism controlling the site of crack initiation in the
hydrofracture tests. The ultimate aim of this work is the development
of a criterion of failure which will permit better control of crack
initiation during hydrofracture testing.
20
8. Conclusions
The results of the hydraulic fracturing tests conducted to date
have established close agreement with earlier measurements of the
magnitude and orientation of the in-situ stress field made using the
overcoring technique. This correspondence suggests that both methods
have the ability to provide useful measurements of the prevailing
in-situ horizontal stress field in granitoids.
Two distinct forms of hydraulic fracture initiation have been
identified:
1) fracture initiation occurring in the isolated test interval
2) fracture initiation occurring under one or both of the inflatable
sealing packers.
Distinctly different styles of pressure records are associated with
these two cases. In both instances it has been shown that it is
possible to undertake satisfactory analysis of the data. When
fractures initiate in the test interval and the net circumferential
stress in the borehole wall is compressive, crack re-opening pressure
provides a reliable means of estimating the maximum horizontal
principal stress magnitude. When fractures initiate under packers
and/or the net circumferential stress in the borehole wall is tensile
or near zero, crack initiation pressure and the laboratory measured
strength can be used to evaluate the major horizontal principal stress
magnitude. In both cases the shut-in pressure provides a reli"able
estimate of the minimum horizontal principle stress magnitude.
The incorporation of a down-hole flow meter in the hydrofracture
system has greatly improved the reliability of determination of crack
re-opening pressure.
21
9. Acknowledgements
The equipment used for the field work was designed and developed
by J. Edgoose of the CSIRO Division of Geomechanics. His considerable
contribution to the program is gratefully acknowledged. Personnel of
the Engineering Services Unit, BMR were responsible for the
construction and commissioning of the down-hole tools, electronics and
handling systems. Particular thanks are due in this regard to R. De
Graaf, E. McIntosh, D. Foulstone and A. Sholtez.
22
10. References
Denham D., L.G. Alexander and G. Worotnicki 1979. Rock Stress
Measurements in the Lachlan Fold Belt, NSW. CSIRO Aust.
Division of Applied Geomechanics, Technical Report No. 84.
Denham D., L.G. Alexander and G. Worotnicki 1980. The Stress
Field Near the Sites of the Meckering (1968) and Calingiri
(1970) Earthquake, Western Australia. Tectonophys, 67:283-317.
Denham D. and L.G. Alexander 1981. Rock Stress Measurements,
Cadoux to Wagin, W.A. CSIRO Aust. Division of Applied
Geomechanics, Technical Report No. 125.
Denham D., L.G. Alexander, I.B. Everingham, P.J. Gregson and
J.R. Enever 1986 (in prep.). Intraplate stress and the 1979
Cadoux Earthquake, Western Australia.
Enever J.R. and B.A. Wooltorton 1983. Experience' with Hydraulic
Fracturing as a Means of Estimating In-situ Stress in
Australian Coal Basin Sediments. In: Hydraulic Fracturing
Stress Measurements, National Academy Press, Washington.
Exner H.E. 1972. Analysis of Grain- and Particle-size
Distributions in Metallic Materials. Int. Metall. Rev.
17:25-42.
Haimson B.C. 1978. Borehole hydrofracturing for the dual
purpose of in-situ stress measurement and core orientation.
In: Proceedings of the Third International Congress of the
International Association of Engineering Geology, Madrid.
Jaeger J.C. and E.R. Hoskins 1966. Stress and failure in rings
of rock loaded in diametral tension or compression, British
Journal of Appl. Phys., 17, 685-692.
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